PV EDUCATION 101: A GUIDE FOR SOLAR INSTALLATION PROFESSIONALS
PV EDUCATION 101:A GUIDE FOR SOLAR INSTALLATION
PROFESSIONALS
Solar is playing an increasingly important role in the transition to a world powered by renewable
energy. Over the past decade, the number of solar installations has grown at an accelerating rate
and with increasing affordability. In the first quarter of 2016, over 29 GW of solar were installed in
the United States.
The price of a solar installation is now less than a third of what it was in 2009, while annual installa-
tions have grown more than tenfold during the same period of time.
As a powerful engine for economic growth, the US solar industry currently employs over 200,000
people—twice as many as in 2010—and now employs more people than the coal, or the oil and gas
industries. As installed capacity continues to increase, SEIA predicts that the U.S. solar workforce
will expand to 420,000 by 2020.
Welcome to the wonderful world of solar energy.
This guide is the ultimate handbook for understanding the world of solar energy. From how a photo-
voltaic system produces energy to mitigating shade losses, this guide contains everything you need
to know to be a solar pro.
INTRODUCTION
A Booming Industry
Why This Guide?
TABLE OF CONTENTS
Solar Basics: Energy, Power, and Irradiance
How a Photovoltaic System Produces Electricity
Solar’s Dark Side: Mitigating Shade
Sizing A PV System
Green Talk: PV System Costs
3
6
10
15
17
Solar Basics: Energy, Power, and Irradiance
Solar panels convert the energy of photons, or light particles, from the sun into electricity. Photo-
voltaic devices, such as solar panels, permit the incoming photons to transfer their energy to
electrons. These energized electrons begin to flow, creating an electric current. We use the terms
irradiance or insolation to refer to the power density of sunlight on a surface.
3
Solar Energy, Power, and Irradiance
Energy from the sunin the form of photonsbeam down to earth
Electrons begin to flow,creating a usableelectric current
Solar panels collectthe photons using silicon,
a semi-conductive material
We typically measure energy in kilowatt-hours (kWh), and power (the rate at which energy is
produced) in kilowatts (kW).
Energy = Power . Time = 1 kW . 1 hour = 1kWh
Figure 2. Annual irradiance value for a 150m2 roof
plane in California. Source: Aurora Solar
In solar, we usually define the size of a solar installation in terms of its power (in kW). Irradiance is
typically reported in units of kilowatt-hours per meter squared per day (kWh/m2-d). The amount of
irradiance hitting the surface of the earth is often quoted in terms of the number of hours of “full-
sun” of solar energy. A "full-sun" is defined as 1 kW/m2.
We can estimate the solar potential of a roof-
top using its area and the local irradiance.
NREL, the National Renewable Energy Labora-
tory, publishes irradiance data in its report
Solar Radiation Data Manual for Flat-Plate and
Concentrating Collectors.
It is fairly straightforward to calculate rooftop
solar potential of a rooftop using this data.
For example, a south-facing roof plane of a
home in California (Figure 2) receives an
average irradiance of approximately 1,900
kWh/m2/year. Dividing the annual irradiance
value by the number of days in a year yields
the average daily irradiance.
4
Solar Resource of a Rooftop
Power
Energy
Irradiance
Quantity Units Definition
Rate of energy production/output
Capacity to do work
Hours of full-sun for a square meter each day
kW
kWh
kWh/m2-d
Table 1. Important quantities used for solar energy
Average Daily Irradiance = 5.2k Wh/m2dayAnnual Irradiance
days/year=
1900k Wh/m2year
365days/year=
5
5
Besides the solar irradiance, Figure 2 also displays information on three additional quantities related
to the solar resource: Solar Access, TOF, and TSRF:
To calculate the amount of solar energy available on a roof face, multiply its area by the average
irradiance value.
If the rooftop has an area of approximately 150m2, the solar energy available on the rooftop is as
follows:
TOF = Energy with actual tilt and orientation
Energy with optimal tilt and orientation
Rooftop Energy [ ] = IrradiancekWh
day[ ] x Area [m2]
kWh
m2 . day
Rooftop Energy = 5.2kWh
dayx 150m2 = 780
kWh
m2 . day
Solar Access = Energy with Shade
Energy without Shade
TOF (Tilt and Orientation Factor):
This is the ratio of the amount of solar
energy a location receives to the amount it
would receive if the orientation of the roof
were optimal.
TSRF (Total Solar Resource Factor):
This is the percentage of the available solar
resource that a location receives as com-
pared to what it would receive with optimal
orientation and without shading. TSRF is
equivalent to the Solar Access multiplied by
the Tilt and Orientation Factor.
Solar Access:
This is the ratio of the actual solar energy
available—taking into account shading
cast by objects in the environment—to
the solar energy that would be available
in the absence of shading. You can learn
more about the effects of shading on PV
systems here
TSRF = Solar Access x TOF
6
How a PhotovoltaicSystem Produces Electricity
As described in Part 1, solar panels convert the energy of photons into electricity. This process is
called the photovoltaic effect.
When a photon hits a photovoltaic device, its energy is transferred from the photon to the local
electrons in the material. These excited electrons begin to flow, producing an electric current.
Solar cells (within solar panels) produce direct current (DC) electricity, which is typically converted
to alternating current (AC) electricity by an inverter, to deliver energy to the grid (which operates
with AC electricity).
It is common practice to refer to all components of a PV system besides the modules as balance of
system (BOS) components. Examples of BOS components include inverters, disconnects, racking,
and wiring.
A simple PV system contains two basic types of components:
How a Photovoltaic System Operates
Components of a Photovoltaic System
Solar Modules: Solar modules contain solar cells that convert sunlight into electricity.
Inverters: A device that converts DC current to AC current.
Organized as 18 modules on a single string
-+1
-+2
-+17
DCDC in
AC
-+18
AC out
InverterPV Modules
Figure 1. Diagram of a simple PV system. Source: Aurora Solar
7
Factors Affecting Solar Photovoltaic System Efficiency
It is important to note that the process of producing electricity from solar energy is not 100%
efficient. Environmental factors, as well as losses in the electrical components, can affect the
efficiency of a PV system. Typical loss categories include:
Temperature
The efficiency of a solar panel varies with temperature. High temperatures have a
negative impact on performance.
Shading
Shading is the obstruction of irradiance due to trees, buildings, terrain, and other
objects in the environment. The effect of shading on the power output of a solar
installation is highly variable. To understand the causes and consequences of shad-
ing, as well as strategies to reduce shading losses, please visit this article.
Wiring and connections
Resistance in the electrical connections of a solar installation typically results in
energy losses of a few percent.
Mismatch
Due to manufacturing variations, modules of the same type can have slightly differ-
ent electrical characteristics. This mismatch between modules can lead to a perfor-
mance loss.
Soiling
Material that accumulates on the surface
of PV panels can block light from reach-
ing the solar cells, thereby reducing the
generated power. The power loss due to
soiling is highly variable, depending on
the type of soiling (such as dust or snow),
and how frequently the panel is cleaned. Figure 2. Soiling, such as dust, on PV modules
reduces power output.
Source: Ferretti and Berghold, PV Tech Power
8
Inverter Efficiency
Converting DC into AC current via an inverter is typically around 96-97% efficient.
Inverters typically have higher efficiency when the DC input power is high. The
conversion efficiency takes a big hit when the input power is much less than the
inverter's rated power.
Age
Solar panels produce less energy the older they get. Typically the decrease in perfor-
mance is assumed to be around 0.5% per year.
Temperature
Inverter Efficiency
Mismatch
Wiring/Connections
Soiling
Age
Shading
Term Typical Value
-0.5%/°C above 25°C
96.5%
98%
98%
95% (highly variable)
-0.5%/year
Highly environment dependent
The above factors are combined in a coefficient called the system derate factor to represent the
overall losses of a solar installation. For instance, PVWatts, an NREL supported PV system energy
production calculator, uses a default system derate factor of 86%. However, depending on the
system design or environmental conditions, this value can be higher or lower.
Table 1. Typical efficiency values.
9
Module efficiency denotes what portion of irradiance a module converts into electricity under stand-
ard test conditions (STC; irradiance of 1000W/m2, ambient temperature of 25°C). As a general rule
of thumb, you can estimate a PV system’s efficiency in converting irradiance into electricity (under
STC) using the following formula:
It is important to note that these are merely back-of-the-envelope calculations. To get an energy
production analysis, you need a software application, such as Aurora, that incorporates all of a PV
system’s environmental, mechanical, and electrical characteristics.
Aurora automatically generates a system loss diagram for any design, which can be manually adjusted if needed.
Overall System Efficiency = Module Efficiency × Derate Factor
10
Solar’s Dark Side:Mitigating Shade
Since PV systems generate electricity based
on the amount of sunlight they receive, it
makes sense that when a shadow is cast on
a panel, for example by a nearby tree, its
power output decreases. However, the
decrease in power could be a lot worse than
it initially seems.
Intuition suggests that power output of the
panel will be reduced proportionally to the
area that is shaded. However, this is not the
case. In his book Renewable Energy and
Efficient Electric Power Systems, Stanford
University’s Gil Masters demonstrates how
shading just one out of 36 cells in a small
solar module can reduce power output by
over 75%.
Effects of Shade on PV Output
Figure 1. Solar panels in partial shade.
Source: lowcarbonlivingblog.wordpress.com
POWER OUTPUT
75%
11
To conceptualize why shading results in such severe losses, it is helpful to use the analogy of water
flowing in pipes. The flow rate of water through the pipe is constant, much like the current through a
cell string is constant for a given irradiance level.
Shading a solar cell is similar to introducing a clog in a pipe of water. The clog in the pipe restricts
the flow of water through the entire pipe. Similarly, when a solar cell is shaded, the current through
the entire string is reduced.
This is significant because every cell in the cell string has to operate at the current set by the shaded
cell. This prevents the unshaded cells from operating at maximum power. Therefore, only a small
amount of shading can have a dramatic effect on the power output of a solar panel.
Similar principles apply to PV modules connected together. The current flowing through an entire
string of modules can be heavily reduced if even just a single module is shaded, leading to poten-
tially significant loss of power output.
Waterflow Analogy
Water Pipe
Clog in pipe
Waterflow
Water Pipe
Clog in pipe
Waterflow
String of Solar Cells
Unshaded solar cell
Electrical curent
Shaded solar cell
String of Solar Cells
Electrical curent
Current Through a String of Solar Cells is Like Water Flowing Through a Pipe
A Shaded Solar Cell is Like a Clog in a Pipe
Figure 2. Analogy of a water pipe to a string of solar cells.
Figure 3. A shaded solar cell is similar to a clog in a water pipe.
Bypass Diodes
Bypass diodes are devices within a module that allow the
current to “skip over” shaded regions of the module. By
utilizing bypass diodes, the higher current of the unshaded
cell strings can flow around the shaded cell string. However,
this comes at the expense of losing the output of the cells
that are skipped over.
Although it would be theoretically ideal to have a bypass
diode for each solar cell, for cost reasons a typical solar
module will have three bypass diodes, effectively grouping
the cells into three series cell strings (Figure 5). For instance,
a 60-cell module will typically have one bypass diode for
every 20 cells.
12
Figure 4. PV arrays with modules connected
in series (left) and in parallel (right).
Fortunately, there are a number of different approaches that can be applied in PV system design to
reduce shading losses. These include the use of different stringing arrangements, bypass diodes,
and module level power electronics (MLPEs).
Stringing Arrangements
Modules connected in series form strings, and
strings can be connected in parallel to an inverter.
The current through all the modules of a string has
to be the same, and the voltage of parallel strings
has to be the same. As we saw in the last section,
a shaded module in a string can bring down the
power output of the string significantly. However,
a shaded module in one string does not reduce
the power output of a parallel string. Therefore, by
grouping shaded modules into separate strings,
the overall power output of the array can be
maximized.
For example, in a commercial system with para-
pet walls, it can be beneficial to group modules
that receive shade from the parapets into strings,
and keep modules that do not receive shade
from the parapets in separate, parallel strings.
This way the unshaded strings can maintain a
higher current and power output.
Approaches to Reduce Shading Losses
Modules in serieson a single string
Modules in paralles strings
Figure 5. PV module containing three
cell strings in series, each with a
parallel bypass diode.
13
Figure 6. Simplified schematic of a PV system utilizing microinverters (left)
and a PV system utilizing DC optimizers (right).
Module Level Power Electronics (MLPEs)
MLPEs are devices that are attached to individual modules in order to increase performance under
shaded conditions (though there are other benefits, such as mismatch mitigation and module-level
monitoring). This is done by performing maximum power point tracking (MPPT) at the module level.
MLPEs include DC optimizers and microinverters.
DC Optimizers
A DC optimizer adjusts its output voltage and current to maintain maximum power without
compromising the performance of other modules.
For instance, when a shaded module produces electricity with a lower current, the DC optimizer
will boost the current at its output to match the current flowing through the unshaded modules;
to compensate, the optimizer reduces its output voltage by the same amount it boosts the
current. This allows the shaded module to produce the same amount of electrical power without
impeding the output of other modules. A system utilizing DC optimizers still needs an inverter to
convert electricity from DC to AC.
Microinverters
As opposed to having a single inverter servicing all of the panels, each panel can have a small
inverter attached to it to convert its output from direct current (DC) to alternating current (AC).
Since each microinverter has an MPPT, and their outputs are connected in parallel, each panel
will operate at its maximum power point, without impacting other panels.
DC AC
PV
Module
DC
DC
Optimizer
PV
Module
DC
DC
Optimizer
PV
Module
DC
DC
Optimizer
AC
DC
DC Optimizer System
Micro
PV
Module
DC
Micro
PV
Module
DC
Micro
PV
Module
DC
AC
Micro Inverter System
14
Effects of MLPEs on PV System Performance
Using Aurora’s simulation engine, we compared the performance of three different PV systems
subject to significant shading.
As shown in Figure 7, we placed a 3.12 kW system near the edge of a roof, which has tall trees
next to it. Note that while this design effectively showcases the performance difference of these
system topologies in shaded conditions, it is not an optimal—or even a practical—design. Our
findings are summarized in Table 1.
Our results show that using MLPEs under these conditions increases system output by 17.3%
annually, showing the benefit of using these components for shade mitigation. Additionally, the
effective yield of a system using a microinverter or a DC optimizer is approximately the same,
although there could be small differences (on the order of 1%) in some cases due to differences
in efficiency curves.
For the same reason that they can mitigate shade losses by decoupling module output, MLPEs
can eliminate module-to-module mismatch losses. These losses are typically caused by manu-
facturing variations that lead to slight differences in the electrical characteristics of two modules
of the same type. Since MLPEs allow the modules to operate independently from one another,
these variations will not impact the system’s overall performance.
Figure 7. The system analyzed for this
case study featured a 3.12 kW system
that is partially shaded by trees.
Table 1. Results from performance simula-
tions of PV system on a California home
utilizing different MLPE components. The
difference between the two MLPE outputs
is attributed to the differences in their
inverters' efficiencies.
Source: Aurora Solar.
String Inverter
Microinverters
DC Optimizers
System
Topology
Annual
Yield
Improvement
with MLPEs
N/A
+17.3%
+17.3%
2,585 kWh/year
3,033 kWh/year
3,035 kWh/year
15
Sizing a PV System
Sizing a PV System from an Electricity Bill
An electricity bill typically reveals information about a residential or commercial customer’s total
monthly energy consumption. From this value alone, it is possible to approximate the required size
of a PV system that offsets monthly energy usage.
Take a hypothetical monthly energy consumption of 500 kilowatt-hours, which is on the lower end
for a household in California. Assuming there are 30 days in a month, an average daily energy use
value can be reached by dividing the monthly use by 30.
Next, insolation values are needed. As mentioned in Chapter 1, insolation values are reported in
kWh/m-day. Since a “full-sun’s” worth of incoming solar energy is approximated as 1 kW/m, insola-
tion values reported in kWh/m-day approximate the hours of full-sun equivalent that a location
receives over the course of a day.
Figure1. Visualization of how total solar insolation received over the course of a day (left) can be represented by number
of full-sun hours (right). Source: pveducation.org
Daily Energy Use = = Monthly Energy Use
Days in Month = 16.7kWh/day
500kWh/mo
30days/mo
Area Under Curve = Solar Insolation
1 kW/m2
So
lar
Rad
iatio
n
Time of Day
1 kW/m2
Time of DayPeak Sun Hours
Equal area under
the two curves
16
For a Californian home, the average daily irradiance value is 5.2 kWh/m-day. By dividing the daily
energy usage by hours a day of full sun, the power output required by the PV system is calculated.
From this analysis, the approximate size of a PV system required to completely offset the average
monthly energy usage of a 500 kWh/month home in California would be about 4 kW.
This would be the size of the PV system required, if our system was 100% efficient. However, that is
not the case because all PV systems have a corresponding derating factor that takes into account
the inefficiencies of the overall system, such as soiling of the panels and imperfect electrical con-
nections.
According to the National Renewable Energy Laboratory’s PVWatts calculator, a typical derate factor
is 0.84. For the sake of this calculation, we assume the derate factor be 80%, or 0.8. In order to
determine the size of the PV system, divide the required power output by the derate factor.
Power Output = = Daily Energy Use
Days hours of full sun = 3.21kW
16.7kWh/day
5.2hours/day
PV System Size = = Power Output
Derate Factor = 4.01kW
3.2kWh
0.8
Figure 2. The California home used
for this PV system sizing exercise.
Source: Aurora Solar
17
Green Talk: PV System Costs
In order to determine financial returns, it is important to have a solid understanding of the basic
economics that dictate PV system costs. There are two general categories of PV systems costs:
capital costs and operation and management (O&M) costs.
Figure 1. NREL PV system cost benchmark summary (inflation adjusted), 2010–2017
Costs Associated with a PV System
Capital costs refer to the fixed, one-time costs of designing and installing the system. Capital costs
are categorized into hard costs and soft costs.
Hard costs are the costs of the equipment, including modules, inverters, and BOS components, as
well as installation-related labor. Soft costs include intangible costs such as permitting, taxes,
customer acquisition costs, etc.
Soft Cost - Others
(PII, Land Acquisition, Sales Tax, Overhead, and Net Profit)
Soft Cost - Install Labor
Hardware BOS - Structural and Electrical Components
Inverter
Module
Capital Costs
18
Incentives and Policies that Benefit Solar Energy
Figure 1 illustrates the relationship between soft and hard costs, and breaks down hard costs into its
components. According to SEIA, while hard costs have come down dramatically over the last
decade, soft costs have remained largely constant.
Cost based incentives, such as the Solar Investment Tax Credit (ITC), allow
those who invest in a solar system to apply a tax credit towards their income tax.
The incentive is determined by the cost of the system, and is independent of its
performance.
O&M costs refer to costs that are associated with running and maintaining the system. These can
include fuel, repairs, and operation personnel. PV systems generally have low O&M costs.
The high capital costs are one of the biggest factors that discourage people from going solar. To
combat this, there are a number of incentives and policies in place to make PV systems financially
competitive.
Operation and Management Costs
Cost-Based Incentives
Performance based incentives (PBIs) encourage PV system owners to install and
maintain efficient systems through payments that are based on the monthly
energy production of the system.
Performance-Based Incentives
In addition to incentives, many states, such as California, implement a net
energy metering (NEM) policy that allows consumers who generate excess
electricity to be reimbursed at the then-prevailing rate of electricity.
For instance, if a residential PV system produces an excess of 100 kWh over the
course of the month, the owner will be reimbursed for 100 kWh at the market
rate of electricity for that time period. The owner is then free to use that reim-
bursement credit towards electricity they consume from the grid when solar is
not meeting their current energy load. Therefore, households with solar PV and
NEM are able to significantly reduce their electricity bill.
Net Energy Metering
TAX
19
Figure 2. Visualized relationship between PV energy production and household electricity use for an average home in New
South Wales, Australia. Source: solarchoice.net.au
In return for a large upfront investment in a solar installation, homeowners that go solar benefit from
a reduced monthly electricity bill. Thus, for NEM regimes the benefit of solar comes in the form of
avoided costs.
For instance, assume that upon installing a rooftop PV system, a home electricity bill is reduced by
$1,500 per year and the cost of the hypothetical PV system is $10,000 after incentives. In order to
calculate the simple payback period, which is the approximate time for a PV system to pay for itself,
we divide the cost of the PV system by the savings.
Thus, the payback period for a system that costs $10,000 and reduces the electricity bill by $1,500
per year is 6.7 years.
Figure 2 shows the relationship between PV electricity production and electricity consumption
during the day. Note that while the PV system can generate more than enough electricity during the
daytime, it can fail to deliver electricity during peak consumption hours.
Basic Financial Calculation for a Residential PV System
Simple Payback Period = = System Cost
Annual Savings = 6.7years
$10,000
$1,500/year
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ow
er
(kW
)
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Home electricityconsumption
Solar PV generation (1.5kW system)
Solar PV generation(3kW system)
An average NSW household in Winter
20
Figure 3. The cumulative (top)
and annual (bottom) cash flows
of a hypothetical PV system.
Source: Aurora Solar
Based on this simple analysis, the system will generate approximately $27,450 in savings over its
lifetime. It is important to note that this is an approximation, and does not take into account factors
such as maintenance costs, changes in electricity price and usage, as well as system degradation
over time.
The figure below shows another financial analysis for a hypothetical residential PV system. In both
graphs, the y-axis is the dollar amount and the x-axis is the year.
The top graph, which shows the cumulative cash flow of the project over time, and indicates that the
project has a payback period of approximately four years. Additionally, the dollar amount in the 25th
year, which is about $25,000, is the cumulative net revenue that the system generated. The bottom
graph is the annual cash flow of the project. The first year is characterized by a large negative cash
flow, due to the large upfront cost required to install the system, but after that there is positive
annual cash flow with the exception to this is in the 14th year, which is when the inverters are being
replaced.
However, a PV system can last much longer than the duration of its payback period. A typical roof-
top PV system has a lifetime of about 25 years. This means that for the last 18 years of its life, after
it has paid itself off, the hypothetical PV system described above will generate revenue in the form of
additional savings. To calculate this revenue, we multiply the annual savings by the remaining
lifetime of the system, after it has paid itself off.
Net Revenue = Annual Savings x Years left in lifetime after system is paid of
Net Revenue = $1,500/year x 18.3year = $27,450
Aurora's solar sales and design software automatically takes into account
everything you've just learned to help you present clear financial information
to customers about their solar purchase.
See how to design and sell
better solar with Aurora
www.aurorasolar.com
Congratulations on finishing your introductory primeron the fundamentals of solar PV!